Relationship between Partial Pressure and Concentration

For a mixture of ideal gases, each component's partial pressure is directly proportional to its molar concentration at a fixed temperature. Starting from the ideal gas equation for one component, A, $P_A V=n_A RT$, dividing both sides by $V$ gives $P_A=\dfrac{n_A}{V}\times RT$. Since $\dfrac{n_A}{V}$ is exactly the molar concentration of A, $[A]$, this gives $P_A=[A]RT$. Applying the same reasoning to every other gaseous species in a reaction gives $P_B=[B]RT$, $P_C=[C]RT$, and $P_D=[D]RT$. This simple substitution -- partial pressure equals concentration times $RT$ -- is exactly the bridge needed to convert the $K_p$ expr …